The invention relates to a method for releasing an airbag in a motor vehicle, in which for the formation of a release decision a plurality of state variables are derived in parallel from an acceleration signal and the airbag is released when a first state variable exceeds a threshold value, influenced by a second state variable, the threshold value being lowered as soon as the second state variable indicates an impact of the vehicle.
A method of this kind is known from European Patent Document EP 458 796 B1 in which restraining means, in particular an airbag, are activated when the integrated output signal of an acceleration sensor exceeds a threshold, the so-called DV threshold. The integrated acceleration represents the velocity change DV, calculated from the start of the impact situation. To adapt to fast and slower crash patterns, various methods for influencing the release characteristics are shown; in particular, the DV threshold is varied as a function of time and/or acceleration. In the case of a high output signal of the acceleration sensor, arming the release readiness is thus achieved through a corresponding lowering of the DV threshold, as a result of which a necessarily very fast release can be carried out in time. To adapt to crash processes with moderate delays, a DV threshold is provided which becomes less sensitive with time, as a result of which, inter alia, a release is prevented if a release of the restraining means would occur too late for a corresponding restraining effect on the passengers. The prevention or desensitization, controlled by a time element, of or for a release is unsatisfactory since no direct physical interrelationship exists with the crash action.
In particular in the case of an offset pole crash with a low degree of overlap, in which no frame side rail of the vehicle is hit, the registered acceleration values lie, for comparable initial speeds, between those of a hard braking involving overrunning the kerbstone (curb) and a head-on crash into a wall. In this area, it is difficult to detect reliably a dangerous impact and to differentiate it from, for example, a hard braking involving overrunning the kerbstone. The release decision therefore requires a greater length of time for the evaluation of the acceleration. While a fast release is of prime importance for fast crash patterns, the release decision for slower crash patterns must also be guided by the consideration that an excessively late release of the airbag has to be prevented so that an occupant whose head has already come very close to the steering wheel is not additionally endangered by the suddenly inflating airbag.
German Patent Document DOS 38 03 426 discloses a method for switching a safety system to be effective, in which the forward displacement of the occupants which occurs during an accident is calculated in the vehicle coordinate system from the signal of an acceleration L pick-up. When the calculated forward displacement exceeds a predeterminable value, a safety device is activated, for example an airbag is inflated. Theoretically such a I method ensures that the airbag is triggered precisely at the right moment. Triggering at a point in time at which the head of the occupant is already hitting the steering wheel would not occur at all. A big disadvantage of this method is, however, that, due to the comparatively great inflation time of the airbag of approximately 30 msec, the decision to release has already to be taken during the initial phase of the crash process, while at this time, however, the forward displacement of the occupants, used as release criterion, can be calculated only very inaccurately from the acceleration values available up to that time.
It is an object of the invention to improve the desensitization of a release unit, shifted to a higher degree of release readiness as a result of impact, for a generic method for airbag release. In consequence, occupants are to be better protected against not only an excessively late release of the airbag but also one which endangers these occupants.
This object is solved by means of the a method of the above referred to type wherein the threshold value is also influenced by a third state variable derived from the acceleration signal and corresponding to the forward displacement of the occupant, by increasing the threshold value as the forward displacement increases. In an advantageous manner, the desensitization of the release unit is coupled to the real crash process by determining the forward displacement of the occupants and using it to control the desensitization of the release unit. Triggering the airbag at a point in time at which the head of the occupant is already. located very close to the steering wheel can be effectively prevented in this way.
While the forward displacement provides no suitable state variable for determining the timing of the release, as described above, it can still be used to control the desensitization which need only set in at a later phase of the crash process. In contrast to the German Patent Document DOS 38 03 426 mentioned above, the forward displacement need not be calculated in advance by a time equivalent to the inflation time of the airbag. It is sufficient to calculate the current forward displacement, which can be determined in a simpler and more accurate way since the desensitization of the armed release circuit can be linked to the simple criterion of whether the current forward displacement exceeds a maximum value above which a release is unwanted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.